Useless, remove. Users changed to check pchr's sentinel, because
that's clearer than comparing the index to P_MDUST or sizeof(pchr) /
sizeof(*pchr).
(pchr): Change sentinel's p_name and p_sname to NULL. Sentinel
checks changed.
I_BAR, I_FOOD, I_OIL, I_LCM, I_HCM, I_UW, I_RAD, I_MAX): Turn macros
into enumeration constants.
(i_type): New. Use where appropriate. No functional changes, except
that I_NONE is now catched properly in a few places.
Store only up to MAXPRCON constituents, not MAXCHRNV; code doesn't
fully support more than three anyway. Remove member p_nv, use item
type I_NONE for unused slots. Rename members p_vtype, p_vamt to
p_ctype, p_camt to avoid confusion with variable-style storage.
(pchr): Initializers adapted.
(nullify_objects, materials_cost, materials_charge, prod,
show_sect_capab, grind): Adapt, simplify where possible, protect
against bad item types in pchr[].
(MAXCHRNV): Unused, remove.
